It is already known from the prior art the centrifugation injection of the aluminum cages in rotors, which are formed by a stack of overlapped annular steel laminations provided with openings that are longitudinally aligned with the openings of other laminations of the stack, in order to define a plurality of axial channels interconnecting the external faces of the end laminations of the stack and which are angularly spaced from each other along a circular alignment, which is concentric to the longitudinal axis of the lamination stack, but radially spaced back in relation to the lateral face of the latter.
The lamination stack, with the longitudinal axis vertically disposed, is positioned inside a mold which defines a lower annular cavity close to the external face of the lower end lamination, and an upper cavity, which is substantially cylindrical or frusto-conical, close to the external face of the upper end lamination and opened to the inlet channel for the admission of aluminum into the mold.
During the aluminum pouring, the lamination stack has its central axial bore, into which will be later mounted the shaft of the electric motor, filled with a core having an upper end substantially leveled with the upper end lamination of the lamination stack, and a widened lower end portion seated on a respective lower end widening of the central axial bore of the lamination stack and against the mold portion that defines the lower cavity.
The aluminum is poured into the upper cavity, passing through the axial channels of the lamination stack to the lower cavity, filling the latter, the axial channels and the upper cavity, in this order, solidifying in a radial inward upward pattern, as the mold rotates around its vertical axis and the metals cools.
Upon completion of the aluminum pouring and solidification, the mold is opened and the formed rotor is submitted to one or more operations to eliminate the inlet channel and unobstruct the adjacent end of the central axial bore of the lamination stack, and to define the correct internal profile for the upper ring of the aluminum cage, which further comprises in a single piece, a lower ring already formed by the mold, and a plurality of bars formed inside the axial channels of the lamination stack.
In the centrifugation injection of these rotors, the upper and lower cavities of the mold and the lamination stack itself are heated, so that the aluminum passes through the upper cavity and through the axial channels of the lamination stack without solidifying, by gravity reaching the lower cavity, filling it and starting to solidify from the outside to the inside, and from the bottom upwardly, while the mold remains rotating.
In order to allow the injection mold, which involves and locks the upper and lower portions of the lamination stack, to rotate around its vertical longitudinal axis, the upper and lower cavities of the mold are mounted, respectively, to an upper bearing and to a lower bearing that are carried by the structure of the injection equipment.
In the molds of the above mentioned type, the deviations of concentricity and parallelism that occur between the axes of the upper and lower cavities cause vibrations in the mold and in the lamination stack during the rotation of the mold, which vibrations actuate in the metallic material being solidified in the upper and lower cavities.
A major problem caused by said vibrations of the rotating mold during the solidification of the aluminum is that the bars of the cage, which are formed inside the axial channels of the lamination stack, and even the rings, tend to present cracks, the bars being transversally broken in the interior of the lamination stack in a manner not perceived by external visual observation of the finished rotor. The rupture or crack of one or more bars or of the upper and lower rings of the cage considerably impairs the quality of the rotor and consequently the efficiency of the electric motor to be formed.
One of the possibilities to minimize or even eliminate the loss of quality by undue vibrations of the mold during the solidification of the aluminum is to mount the two cavities of the mold on a single lower bearing, whereby the axes of the two mold parts are united. However, in this solution, the upper and lower cavities of the mold are guided by columns affixed to the lower cavity. The upper cavity is axially displaced, guided by the columns, to open and close the mold, whereby the upper cavity is retained in a sliding relationship with the columns, considerably limiting the automation of the operations of loading the lamination stack in the mold and extracting the centrifuged rotor, besides causing problems of concentricity and rotor strike.
While the mounting of the two mold cavities on a single lower bearing assembly allows eliminating the problem of cracks in the parts of the aluminum cage caused by deviations of concentricity and parallelism between the axes of the two mold cavities, this known prior art mold still maintains the upper mold cavity mounted to the columns which are axially and eccentrically projected from the lower cavity, when said upper mold cavity reaches the open mold position for the loading of the lamination stack or removal of the centrifuged rotor. Thus, the movement of the lamination stack in and out of the mold must be effected by passing the lamination stack radially through the gap formed between two consecutive columns. This characteristic of the solutions in which there is only one lower bearing and the upper cavity is axially displaced along the columns between the open and closed mold positions requires complex solutions to reach a high degree of automation in the production of the rotors with a short cycle time, impairing the productivity.